Over the decades, wireless communication systems have become more and more technologically advanced, with performance increasing in terms of smaller size, operation at higher frequencies and the accompanying increase in bandwidth, lower power consumption for a given power output, and robustness, among other factors. The trend toward better communication systems puts ever-greater demands on the manufacturers of these systems.
Today, the demands of satellite, military, and other cutting-edge digital communication systems are being met with microwave technology, which typically operates at frequencies from approximately 500 MHz to approximately 60 GHz or higher. Many of these systems use couplers, such as directional couplers, in their microwave circuitry.
Traditional couplers, especially those that operate at lower frequencies, typically require a relatively long parts housing size (i.e., a long packaging size) since coupling between lines is often required over a long distance.
Popular technologies for microwave technologies include low temperature co-fired ceramic (LTCC), ceramic/polyamide (CP), epoxy fiberglass (FR4), fluoropolymer composites (PTFE), and mixed dielectric (MDk, a combination of FR4 and PTFE). Each technology has its strengths, but no current technology addresses all of the challenges of designing and manufacturing microwave circuits.
For example, multilayer printed circuit boards using FR4, PTFE, or MDk technologies are often used to route signals to components that are mounted on the surface by way of soldered connections of conductive polymers. For these circuits, resistors can be screen-printed or etched, and may be buried. These technologies can form multifunction modules (MCM) which carry monolithic microwave integrated circuits (MMICs) and can be mounted on a motherboard.
Although FR4 has low costs associated with it and is easy to machine, it is typically not suited for microwave frequencies, due to a high loss tangent and a high correlation between the material's dielectric constant and temperature. There is also a tendency to have coefficient of thermal expansion (CTE) differentials that cause mismatches in an assembly. Even though recent developments in FR4 boards have improved electrical properties, the thermoset films used to bond the layers may limit the types of via hole connections between layers.
Another popular technology is CP, which involves the application of very thin layers of polyamide dielectric and gold metalization onto a ceramic bottom layer containing MMICs. This technology may produce circuitry an order of magnitude smaller than FR4, PTFE, or MDk, and usually works quite well at high microwave frequencies. Semiconductors may be covered with a layer of polyamide. However, design cycles are usually relatively long and costly. Also, CTE differentials often cause mismatches with some mating assemblies.
Finally, LTCC technology, which forms multilayer structures by combining layers of ceramic and gold metalization, also works well at high microwave frequencies. However, as with CP technology, design cycles are usually relatively long and costly, and CTE differentials often cause mismatches with some mating assemblies.
Advances have been made in reducing the size of LTCC couplers and FR4 couplers, by using strip-line spiral-like configurations. Examples of spiral-like configurations for couplers using various technologies may be found in U.S. Pat. No. 3,999,150 to Caragliano et al., U.S. Pat. No. 5,689,217 to Gu et al., and U.S. Pat. No. 5,841,328 to Hayashi, all incorporated herein by reference. However, using spiral-like configurations for couplers based on these technologies have certain limitations, as described below.
Hard ceramic materials may provide dielectric constants higher than approximately 10.2, but components utilizing these materials cannot be miniaturized in a stand-alone multilayer realization. For example, bond wire interconnects must be used for the realization of microstrip circuitry, increasing the overall size of the resulting microwave devices. Other ceramic materials have limited dielectric constants, typically approximately 2 to 4, which prevent close placement of metalized structures and tend to be unreliable for small, tight-fitting components operating at microwave frequencies. Additionally, ceramic devices operating at microwave frequencies may be sensitive to manufacturing limitations and affect yields. LTCC Green Tape materials tend to shrink during processing, causing mismatches preventing manufacturers from making smaller coupling lines and placing coupling lines too closely lest they lose their spacing due to shifting during processing. For these reasons, spiral-like configurations of couplers cannot be too compact and the benefits of using spirals are limited.
FR4 materials have other disadvantages. For example, FR4 materials have a limited range of dielectric constants, typically approximately 4.3 to 5.0, preventing manufacturers from placing metalized lines too compactly. Manufacturers utilizing this material also cannot avail themselves of the advantage of fusion bonding. Additionally, FR4 materials are limited in the tolerance of copper cladding that they can sustain—typically 1.4 mils is the minimum thickness, so the dimensional tolerances are limited. As with ceramics, spiral-like configurations of couplers cannot be too compact, and the benefits of using spirals are limited for FR4. MDk materials also have similar disadvantages to FR4.
PTFE composite is a better technology than FR4, ceramics, and MDk for spiral-like couplers. Fluoropolymer composites having glass and ceramic often have exceptional thermal stability. They also allow copper cladding thickness below approximately 1.4 mils, which permits tighter control of etching tolerances. Additionally, these materials have a broad range of dielectric constants—typically approximately 2.2 to 10.2. Also, they can handle more power than most other material. All these features allow spiral-like couplers to be built much more compactly on PTFE than is possible using other types of material. Furthermore, complex microwave circuits can be fabricated using PTFE technology and the application of fusion bonding allows homogeneous multilayer assemblies to be formed.